There has been known a technique in which a discharge electrode is arranged in an exhaust passage of an internal combustion engine, and a corona discharge is caused to occur from the discharge electrode, whereby particulate matter (hereinafter also referred to as PM) is charged and condensed or aggregated (see, for example, a first patent document). By the condensation or aggregation of the particulate matter, the number of particles in the particulate matter can be decreased. Moreover, the sizes of particles in the particulate matter become large, so when a filter is arranged at a downstream side, it becomes easy to trap the particulate matter with the filter.
In addition, there has also been known a technique in which when an electric current passing through a discharge electrode is equal to or more than a predetermined value, it is determined that particulate matter has adhered to the discharge electrode, and an applied voltage to the electrode is caused to increase so as to remove the particulate matter from the discharge electrode (see, for example, a second patent document).
Moreover, there has further been known a technique in which an insulator is arranged between an electrode and a housing on which this electrode is mounted so as to prevent electricity from flowing therebetween, wherein in cases where an average current in a predetermined period of time at the time when an inspection voltage, being about a fraction of a corona discharge voltage, is applied to the electrode is equal to or greater than a predetermined value, a determination is made that the insulation performance of the insulator is in a dropped state due to the adhesion of particulate matter (see, for example, a third patent document).
With this technique, a flow of electricity through substances adhered to a surface of the insulator is detected. Here, even in cases where a voltage, which is lower than a voltage at which a strong electric discharge such as a corona discharge or the like occurs, is applied to an electrode, electricity can flow through substances floating in an exhaust gas. However, in the past, no consideration has been given to an electric current which is detected at the time when electricity flows through the substances floating in the exhaust gas. In addition, the electric current at this time has not been utilized.
Here, note that there has also been known a technique in which in an engine where ignition timing is retarded immediately after starting thereof, the air fuel ratio of a mixture is controlled in a feedback manner, while detecting HC in an exhaust passage by the use of an ion sensor arranged therein (see, for example, a fourth patent document). However, assuming that both the ion sensor and a particulate matter processing apparatus are provided together, an increase in costs will result, and at the same time, a space for arranging both of them will have to be obtained.